I would Just provide the balance from that active account in addition to the 500K account to emphasize your financial independence. No need to explain where it has come from as you are not seeking DTV sponsorship.
Overstayers have been approved for the DTV. 500k should be in a personal bank account. Vietnam is among the top embassies to apply from which speaks to the ease of its' application process. London is also very "friendly" with its' application process.
Anonymous participant Yes, just be patient. You won't have to wait as long as you have to get your DTV, which is the good thing. Explore Cambodia while you wait, it's a nice country.
It has taken 6 weeks to get approved in Cambodia. Many around a month before current delay. You not being asked for any further docs is a good sign your docs are in order.
Tiger Eye is a solid option and many DTVERS approved via them. Muay Thai is the course they offer for DTV approval but you get access to yoga etc. less than a 6 month plus course often leads to a rejection.